If you search the forum for other discussions about this, you'll see that having custom search tabs is not very realistic. The search results are scraped with regexes that are unique to each site, among other things, and there isn't a practical way for users to do all of this - without a significant re-write of the code, I would imagine.
To answer your other question: there is no plugin architecture for Alt.Binz.
Edit: Only NZBIndex isn't working in 0.28.5, beta.binaries.nl site is down. All other search engines are AOK, AFAIK.